Zeotropic blends have concentrations of constituents that are different in the liquid and vapor phases at equilibrium. A consequence of this is that the dew point temperature (temperature at which condensation first begins) is different from the bubble point temperature (temperature at which evaporation or bubble formation first begins) at a given pressure. The difference between the dew and bubble points is termed the “temperature glide”. This temperature glide affects the temperature differences that drive heat transfer in a heat exchanger. In addition, there are mass transfer effects because of the differences in liquid and vapor phase concentrations of constituents. It is generally found that refrigerant blends with large temperature glides are not suitable for use in shell-and-tube heat exchangers.
Blends that are zeotropic in nature are more common than azeotropic blends, and it may be more common to find a zeotropic blend that will match the characteristics of refrigerant R123. To match the capacity of R123 with a zeotropic blend requires one component to be lower pressure (higher boiling point) than R123 and the other to be higher pressure (lower boiling point). For lower pressure refrigerants, R1336mzz(Z) is the only new low GWP HFO known to be under serious development for commercial application as a refrigerant and foam blowing agent. Generally, the farther apart the normal boiling points are of the two blend constituents, the greater will be the temperature glide.